Thermal property of ionic clathrate hydrates and their applications to solid electrolytes of electrochemical systems

Abstract

Ionic clathrate hydrates (ICHs) are the types of non-stoichiometric crystalline inclusion compounds that are formed by ionic interaction between water-ionic host and relatively hydrophobic ionic guests. The water molecules are linked through hydrogen bonding and they form a water-ionic framework, namely, “cage” that can stably accommodate the ionic guest. ICHs have been investigated for applications to many industrial fields of energy and environment, such as gas storage, selective separation of targeted component especially carbon dioxide from flue gas. Additionally, there have also been some efforts to use ICHs as solid electrolytes of electrochemical systems due to their high ionic conductivity even at low temperature. Because they are considered as novel types of proton conductors, further intensive study about the crystal structure, host-guest interaction, electrochemical properties and practical application to electrochemical system should be performed. In this study, spectroscopic analysis of thermal expansivity, host-guest interaction, electro-chemical stability and electrochemical application of ICHs were intensively investigated.First Topic (Thermal Expansivity of Ionic Clathrate Hydrates Including Gaseous Guest Molecules): Although thermal expansion is a key factor in relation to the host-guest interaction of clathrate hydrates, few studies have investigated the thermal behavior of ICHs. The existence of ionic species in these hydrates cre-ates a unique host-guest interaction compared to that of non-ionic clathrate hydrates. It was revealed that X-ray diffraction cannot be used for research of tetramethylammonium hydroxide clathrate hydrates due to damage of the cations by the X-ray, which results in abnormal thermal expansion of the ICHs. Hence, in the present work, the thermal expansivities of binary sII Me4NODo16D2O and sI DClO4o5.5D2O were measured by neutron powder diffraction (NPD) in order to shed light on their thermal behavior. General corr...